Control of phasic aortic pressure in experimental dogs is achieved with a feedback pressure control system. The system is designed to either change pressur during several beats in an arbitrary manner, or to reproduce a pressure waveform over an extended period of time. Typical use is for evaluating effects of drug intervention where the pressure waveform before the intervention is reproduced after the intervention, thus allowing changes in systemic pressure. A modification of the system will be used to control end diastolic filling pressure.